Lead exposure may be on the decline, but it’s still taking its toll on children’s performance in school.

Legal requirements to remove lead from gasoline, paint and other common products have led to decreases in lead exposure. But remnants of the metal remain, according to the latest study, and this legacy may be enough to affect children’s cognitive functions.

Lead poisoning is still a concern for American families, especially those living in urban areas where older housing materials remain sources of potential exposure. A nine-year study of over 367,000 Detroit children published in the American Chemical Society’s journal Environmental Science & Technology shows that blood levels of lead fluctuate with the seasons among kids, primarily due to dust they breath that is contaminated with lead.

While the primary sources of lead have been eliminated, the researchers report that cities still retain a “legacy” of the contamination in discarded water pipes or paint, and contaminated particles that are swept up from soil and into the air are causing an rise in blood lead levels in kids by anywhere from 11% to 14% during July through September as compared to January.

Continued exposure to lead can have detrimental effects on children’s development, according to a separate study published in the American Journal of Public Health. Researchers reported that early childhood lead exposure was linked to low performance in math, science and reading in elementary and junior high students—even at exposure levels lower than the federal limit. “Despite a dramatic decline in blood lead concentrations, childhood lead poisoning continues to be the most important and preventable environmental problem among children and contributes signiﬁcantly to the burden of childhood diseases,” the authors write.

In the study, the scientists studied blood lead levels in 21,281 kids who had been tested before age six between 1990 and 2008. They then compared these levels to their math, science and reading scores on the Michigan Education Assessment Program tests from 2008-2010. They found that high blood levels before age six was associated with low academic performance in grades 3, 5 and 8.

“In reality, there is no well-documented threshold for acceptable levels of lead in the body, and our research shows that in amounts as small as 2-5 micrograms per deciliter, children had significant cognitive impairment,” said study author Michael Elliott, a professor of biostatistics at the School of Public Health and a research professor at the Institute for Social Research, in a statement.

Recognizing the growing body of evidence linking lead exposure to behavioral and physical problems, in addition to cognitive impairments, in May 2012 the Centers for Disease Control adjusted the threshold for lead poisoning from 10 micrograms of lead per dL of blood to 5 mcirogm/dL or higher.

Both studies address the need for protecting children from lead exposure, and the higher risk for children living in cities. “As public health resources dwindle in cities and states, the focus on preventing and eliminating childhood lead poisoning is placed low on the public health and education agenda. This becomes increasingly problematic as a signiﬁcant number of properties in heavily lead-polluted cities have been poorly maintained because of the national housing crisis,” the authors of the academic performance study write.

Understanding that children continue to be exposed to potentially harmful levels of lead, despite its elimination from its most common sources, should add a sense of urgency to continued efforts to address lead as a public health issue, and better focus intervention efforts to limit exposure.